Q: What may be making this noise ? on 2006 Nissan Xterra

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my xterra has 79,600 miles on it and started making this intermittent noise-like it was going over rumble strips.low sound at first coming from rear of vehicle can feel vibration sitting in back seat. had local mechanic look at it they decided it was spider gears in rear end-replaced gears and fluid. sound is now louder and more often than before.had back for 2 days.
(2) Answers
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I have an '02 xterra with 62K on it and this same noise started for me less than a week ago.... I described it to a mechanic friend as a intermittent washboard gravel road. Did your radiator fix solve your problem for good... can you give more details... Is this a dealer fix or could it be a do it yourself.
Don't know if you could do this or not, i couldn't. It seems to have gone away, but dealer said it may take some time for conditioners to work it all out. I put over 400 miles on it this weekend and heard it less each day,not at all last 2 days.
Took to dealer,not good news if left go.Internal transmition cooler in radiator leaking.When leak occurs
cooling system takes on tranny fluid and level in tranny gets low-check radiator by pulling cap-you'll know right away if that's the problem-milky film in my radiator. Must be common problem knew to check before we even test drove. Fix was to flush and fill cooling system,flush and clean tranny twice then fill and bypass internal cooler.Tried to get me to by new radiator i said why-cleaning everything then bypassing leak.About $500 to repair.Noise gone since leaving dealer,we'll see.